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United States Patent |
6,241,807
|
Enick
,   et al.
|
June 5, 2001
|
System for recovery of metals from solutions thereof
Abstract
A system for recovery of metals from solutions containing dissolved metals.
The system includes an apparatus and method for placing supercritical
carbon dioxide that contains an extractant in contact with the solution
and agitating the two resulting phases. Once the metals are extracted by
the extractant, they can be removed from the carbon dioxide phase by
depressurization or by reduction by exposure to hydrogen. The extractant
preferably comprises a metal binding group, a spacer group and a C0.sub.2
-philic group.
Inventors:
|
Enick; Robert M. (Pittsburgh, PA);
Beckman; Eric (Aspinwall, PA)
|
Assignee:
|
University of Pittsburgh (Pittsburgh, PA)
|
Appl. No.:
|
265319 |
Filed:
|
March 9, 1999 |
Current U.S. Class: |
75/721; 75/743; 75/744 |
Intern'l Class: |
C22B 003/26 |
Field of Search: |
75/721,744,743
210/634
|
References Cited
U.S. Patent Documents
4171282 | Oct., 1979 | Mueller | 252/356.
|
5246507 | Sep., 1993 | Kodama et al. | 148/250.
|
5356538 | Oct., 1994 | Wai et al. | 210/634.
|
5728431 | Mar., 1998 | Bergbreiter et al. | 427/388.
|
Primary Examiner: King; Roy
Assistant Examiner: McGuthry-Banks; Tima
Attorney, Agent or Firm: Conley, Rose & Tayon P.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims the benefit of provisional application Ser.
No. 60/077,462, filed Mar. 10, 1998, entitled System for Recovery of
Metals from Solutions Thereof; and is also a continuation-in-part of
application Ser. No. 09/250,537, filed Feb. 16, 1999, entitled Method and
Composition for Surface Treatment of Metals now issued as U.S. Pat. No.
6,183,810; which is a continuation-in-part of application Ser. No.
08/831,999, filed Apr. 1, 1997, entitled Further Extraction of Metals in
Carbon Dioxide and Chelating Agents Therefor, and now issued as U.S. Pat.
No. 5,872,257; which is a continuation-in-part of Ser. No. 08/233,105,
filed Apr. 1, 1994, entitled Extraction of Metals in Carbon Dioxide and
Chelating Agents Therefor and now issued as U.S. Pat. No. 5,641,887. All
of the aforementioned applications are incorporated herein in their
entireties.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
Claims
What is claimed is:
1. A method for extracting a metal from an aqueous solution containing the
metal, comprising:
(a) dissolving an extractant compound in CO.sub.2 to form an extractant
solution, wherein the extractant compound comprises a CO.sub.2 -philic
group selected from the group consisting of silicones, phosphazenes, and
alkylene oxides;
(b) contacting the extractant solution with the aqueous solution
sufficiently to allow the extractant to capture the metal; and
(c) removing the metal from the extractant solution.
2. A method for extracting a metal from an aqueous solution containing the
metal, comprising:
(a) dissolving an extractant compound in CO.sub.2 to form an extractant
solution, wherein the extractant compound comprises a CO.sub.2 -philic
group having the formula --(CF.sub.2 CF(CF.sub.3)O).sub.n CF.sub.2
CF.sub.2 where n is 2 to 25;
(b) contacting the extractant solution with the aqueous solution
sufficiently to allow the extractant to capture the metal; and
(c) removing the metal from the extractant solution.
3. A method for extracting a metal from an aqueous solution containing the
metal, comprising:
(a) dissolving an extractant compound in CO.sub.2 to form an extractant
solution, wherein the extractant compound comprises an alkyl spacer having
the formula (CH.sub.2).sub.x where x is at least 2;
(b) contacting the extractant solution with the aqueous solution
sufficiently to allow the extractant to capture the metal; and
(c) removing the metal from the extractant solution.
4. A method for extracting a metal from an aqueous solution containing the
metal, comprising:
(a) dissolving an extractant compound in CO.sub.2 to form an extractant
solution;
(b) contacting the extractant solution with the aqueous solution
sufficiently to allow the extractant to capture the metal; and
(c) removing the metal from the extractant solution;
wherein the extractant compound comprises a CO.sub.2 -philic group, an
alkyl spacer having the formula --(CH.sub.2).sub.x --, and a metal-binding
group and the CO.sub.2 -philic group is selected from the group consisting
of silicones, phosphazenes, and alkylene oxides.
5. The method according to claim 4 wherein the metal binding group is an
organic functional group.
6. The method according to claim 4 wherein the metal-binding group contains
a protonatable nitrogen, oxygen, or sulfur.
7. The method according to claim 4 wherein the spacer group comprises
(CH.sub.2).sub.x where x is at least 2.
8. A method for extracting a metal from an aqueous solution containing the
metal, comprising:
(a) dissolving an extractant compound in CO.sub.2 to form an extractant
solution;
(b) contacting the extractant solution with the aqueous solution
sufficiently to allow the extractant to capture the metal; and
(c) removing the metal from the extractant solution;
wherein the extractant compound comprises a CO.sub.2 -philic group, an
alkyl spacer having the formula --(CH.sub.2).sub.x --, and a metal-binding
group and the CO.sub.2 -philic group has the formula --(CF.sub.2
CF(CF.sub.3)O).sub.n CF.sub.2 CF.sub.2 where n is 2 to 25.
9. The method according to claim 8 wherein the metal binding group is an
organic functional group.
10. The method according to claim 8 wherein the metal-binding group
contains a protonatable nitrogen, oxygen, or sulfur.
11. The method according to claim 8 wherein the metal-binding group is
selected from the group consisting of ethers, esters, ketones, and
carboxylic acids.
12. A method for extracting a metal from an aqueous solution containing the
metal, comprising:
(a) dissolving an extractant compound in CO.sub.2 to form an extractant
solution;
(b) contacting the extractant solution with the aqueous solution
sufficiently to allow the extractant to capture the metal; and
(c) removing the metal from the extractant solution;
wherein the extractant compound comprises a CO.sub.2 -philic group, a
metal-binding group and an alkyl spacer having the formula
--(CH.sub.2).sub.x -- where x is at least 2.
13. The method according to claim 12 wherein the metal binding group is an
organic functional group.
14. The method according to claim 12 wherein the metal-binding group is
selected from the group consisting of ethers, esters, ketones, and
carboxylic acids.
15. The method according to claim 12 wherein the metal-binding group
contains a protonatable nitrogen, oxygen, or sulfur.
16. The method according to claim 12 wherein the CO.sub.2 -philic group has
the formula --(CF.sub.2 CF(CF.sub.3)O).sub.n CF.sub.2 CF.sub.2 where n is
2 to 25.
17. The method according to claim 12 wherein the CO.sub.2 -philic group is
--(CF.sub.2).sub.m CF.sub.3 where m is 2 to 10.
Description
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to novel compounds for extracting
precious metals from solution, and more particularly to compounds useful
for extracting precious metals into supercritical CO.sub.2. Still more
particularly, the present invention relates to compounds comprising three
part molecules in which the head is a ligand that forms a complex with the
desired metal, the body is a spacer group, and the tail is a CO.sub.2
-philic compound that renders the molecule soluble in CO.sub.2.
2. Background of the Invention
In recent years, a growing demand for precious metals in high-technology
applications and the increasing cost of precious metals has made recovery
of these metals very important. To meet these demands, industry is turning
to new sources of precious metals such as complex sulfide ores, and
recycling precious metals from catalysts and electronic scrap.
Conventional solvent extraction is sometimes referred to as liquid ion
exchange extraction or liquid/liquid extraction. This process comprises
two steps. In the first, the extraction step, dilute aqueous feed solution
which contains the metal ion to be recovered is contacted with an organic
diluent or carrier containing an ion exchanger or ligand dissolved
therein. The organic carrier is typically a hydrocarbon and is immiscible
in water. The resulting metal complex migrates to the organic phase. In
the second, the stripping step, the separated "loaded" organic phase is
mixed with an aqueous solution of a stripping agent and the procedure is
reversed, with the metal ion passing back to the new aqueous phase. Thus,
the dilute feed solution is converted into a highly concentrated solution,
from which the metal values are more readily recovered. The barren organic
phase can then be recycled through the system.
The system described above has serious drawbacks, however, as the organic
carrier is not typically environmentally friendly and the large volumes of
water contaminated through contact with the carrier create a sizable
disposal problem. Hence it is desired to provide a carrier or solvent that
is capable of performing the same extraction without the negative
environmental ramifications.
Supercritical carbon dioxide is environmentally innocuous, as well as being
inexpensive and safe to handle. Carbon dioxide has elicited significant
scientific interest over the past 15 years because it is considered a
"green" alternative to conventional organic solvents. CO.sub.2 is
inexpensive (approximately $80/ton, 1-2 orders of magnitude less than
conventional solvents), non-flammable, and is not currently regulated as a
volatile organic compound (VOC).
Although CO.sub.2 possesses distinct advantages as a solvent, it also
exhibits three significant disadvantages, which have limited current
commercial applications, for the most part, to food processing and polymer
foam production. First, use of CO.sub.2 (in either the liquid or
supercritical state) requires the use of elevated pressures, as the vapor
pressure of CO.sub.2 at room temperature is over 900 psi. Consequently,
design and construction of equipment is significantly more expensive than
for 1 atmosphere analogs.
Second, utility costs due to processing with high pressure CO.sub.2 can be
prohibitively high, in particular those due to gas recompression.
Consequently, while it has been suggested that depressurization of a
CO.sub.2 solution to 1 atmosphere is an easy route to recovery of
products, it is not likely that a CO.sub.2 -based process will be
economically viable if extensive depressurization is used to recover
dissolved products.
The final significant obstacle to the use of CO.sub.2 as a solvent in
conventional chemical processes is its low solvent power. Although its
solvent power was once suggested to be comparable to that of liquid
alkanes, recent research has shown that this generalization is in error.
Calculation produces solubility parameters for CO.sub.2 of 4-5
cal/cm.sup.3 in the liquid state, similar to those of fluorinated
materials and slightly lower than those of silicones. It is generally
accepted that CO.sub.2 will not solubilize significant quantities of
polar, high molecular weight, or ionic compounds. The low CO.sub.2
-solubility of many compounds of interest means that large volumes of
CO.sub.2 are required in a potential process, further diminishing the
chance for favorable economics.
Hence, it is presently desired to provide an environmentally friendly
system for recovering precious metals. The system should be cost-effective
and non-hazardous. Thus it is further desired to provide an extractant
that is capable of complexing with the desired metal(s) and is soluble in
supercritical or liquid CO.sub.2.
BRIEF SUMMARY OF THE INVENTION
The present invention comprises a system for recovering precious metals
from acidic solutions thereof. The present system is environmentally
friendly, cost-effective and non-hazardous. The present invention
comprises dissolving a metal-binding compound (extractant) in carbon
dioxide and contacting the CO.sub.2 solution with an aqueous solution
containing dissolved metals. The aqueous solution is typically an acidic
solution in which the metals are present as chlorates. The extractant
binds with the metal atoms, transferring them into the CO.sub.2 phase. The
preferred system further includes recovery of the metals from the CO.sub.2
phase by exposure to hydrogen.
The extractants of the present invention comprise highly CO.sub.2 -soluble
molecules that are effective for extracting precious metals from solutions
containing the precious metals. The extractant molecules are designed such
that they exhibit miscibility with CO.sub.2 at moderate pressures, and the
resulting complexes between the extractant and the metals in question also
exhibit miscibility with CO.sub.2 at moderate pressures.
The extractants of the present invention contain certain metal binding
groups that contain oxygen, nitrogen or sulfur. These metal binding groups
are protonated when the CO.sub.2 -phase in which they are dissolved is
placed in contact with an acidic aqueous phase, as is the case during the
extraction of precious metals from HCl-based leach solutions. The
protonated extractants can then bind the precious metal anions (of the
form MCl.sub.x.sup.-2 where M is a precious metal such as platinum, gold,
palladium, rhodium, etc.) and transfer them from the aqueous phase to the
CO.sub.2 phase, from which they can be recovered. The metal binding group
is selected on the basis of the metal to be recovered and is rendered
soluble in CO.sub.2 by the addition of a CO.sub.2 -philic tail. To
minimize the effect of the CO.sub.2 -philic tail on the metal binding
group, a spacer group is included in the extractant molecule between the
CO.sub.2 -philic tail and the metal binding group.
DETAILED DESCRIPTION OF THE INVENTION
The present invention comprises CO.sub.2 -soluble, three-part molecules
that can be used to extract precious metals from solutions containing the
precious metals. The three parts are: a CO.sub.2 -philic group, an alkyl
spacer (--(CH2)x--), and an organic functional group containing a
protonatable nitrogen, oxygen, or sulfur. Compounds having this
configuration can be used to extract metal anions of the form
MCl.sub.x.sup.-2 from aqueous solutions.
CO.sub.2 -philic Groups
To insure high solubility in CO.sub.2 at moderate pressures, the
extractants include certain functional groups that interact favorably, in
a thermodynamic sense, with carbon dioxide. These CO.sub.2 -philic
functional groups include fluoroalkyls (--CF.sub.2 --), fluoroethers
(--CF.sub.2 --CF(CF.sub.3)--O--; --CF.sub.2 --CF.sub.2 --O--), silicones
(--Si(R).sub.2 --O--), phosphazenes (--P(R).sub.2.dbd.N--), and alkylene
oxides (--CH.sub.2 --CH(R)--O--), where R is a group except hydrogen.
Varying numbers of the CO.sub.2 -philic groups can be used to render the
desired extractant soluble in CO.sub.2. According to a preferred
embodiment, at least three units of a fluoroether such as
hexafluoropropylene oxide are used as the CO.sub.2 -philic group. In
alternative preferred embodiments, at least six units of a silicone
functional group, at least six units of a fluoroalkyl functional group, at
least three units of a fluorinated polyacrylate functional group, or at
least six units of a phosphazene functional group are used. It will
nevertheless be understood that more or fewer than these numbers of units
can be used without departing from the scope of the present invention.
Spacer Group
The CO.sub.2 -philic groups preferred in the present invention tend to be
strong electron-withdrawing agents. Thus, proximity to the metal binding
group mitigates the ability of the metal binding group to perform its
intended purpose. Hence, in cases where fluoroethers or fluoroalkyls are
used the CO.sub.2 -philic group, a non-electron withdrawing spacer group
is preferably included between the metal binding group and the CO.sub.2
-philic group. The spacer group is preferably a (CH.sub.2).sub.x group in
which x is preferably at least 3. While as few as 1 or 2 spacer groups can
be used, and while there is no particular upper limit on the number of
spacer groups, it has been found that fewer than 2 spacer groups do not
typically provide sufficient isolation from the electron-withdrawing
CO.sub.2 -philic groups and more than 25 spacer groups render the
extractant molecule unnecessarily cumbersome.
Metal Binding Group
As mentioned above, the extractants of the present invention include a head
group constituent that can be readily protonated, such as N, O, or S. In
general, oxygen-containing solvents can extract gold, while compounds
containing nitrogen or sulfur atoms are required to extract platinum and
palladium. The protonatable head group constituent is protonated upon
exposure of the extractant to the acidic aqueous solution. Once
protonated, the extractant molecules bind the precious metal anions in the
solution, thereby transferring the metal atoms from the aqueous to the
CO.sub.2 phase. Alternatively, the metal-binding group can be selected
from the oxygen-bearing compounds, such as ketones, ethers, etc., which
bind to gold, although they are less preferred as binders for platinum or
palladium
Recovery of Metals
In one type of conventional recovery system, metal ions are leached or
dissolved out of ore or mineral waste, resulting in an acidic aqueous
solution in which the metals are present as chlorates. According the
present system, a CO.sub.2 -soluble extractant having a metal binding
group selected to bind the desired metal is dissolved in supercritical
CO.sub.2. The supercritical CO.sub.2 is then contacted with the aqueous
solution containing the metal to be recovered. Because supercritical
CO.sub.2 is not miscible with water, a liquid--liquid system is formed.
Thorough contact can be ensured by agitating the liquid--liquid system, as
is known in the art. The contact results in the capture of the metal by
the metal binding group of the extractant and thus transfer of the metal
into the CO.sub.2 phase.
Once the precious metal anions are extracted into the CO.sub.2 phase, the
precious metal can be recovered by depressurization of the CO.sub.2
followed by reduction of the metal to zero-valent state using conventional
methods. A more efficient and thus more preferred method is to reduce the
metals while still in solution by exposure to hydrogen. Hydrogen is
miscible with carbon dioxide at elevated pressures and its use allows
reduction and recovery of the metal without depressurizing the CO.sub.2,
which in turn leads to savings in utility costs.
The Examples that follow are merely illustrative of currently preferred
embodiments and are not intended to limit the present disclosure, the
scope of which is commensurate with the claims that follow.
EXAMPLE 1A
Preparation of Tertiary Amine Extractants
A fluoroalkyl-functional CO.sub.2 -soluble tertiary amine extractant was
generated as follows. An amount of 3-(dibutylamino)propylamine was
dissolved in previously dry Freon 113 (1,1,2 trichlorotrifluoroethane), to
which was then added pyridine at a 1:1 molar ratio to the amine.
Subsequently, perfluorooctanoyl chloride, 1:1 molar ratio to the amine, in
dry Freon 113 was added while stirring under nitrogen. After stirring at
room temperature for several hours, the Freon 113 solution was extracted
with water to remove the pyridine hydrochloride byproduct. The solvent was
then removed under vacuum and the product recovered in greater than 95%
yield. On the IR spectrum the shift of the carbonyl peak from 1776
cm.sup.-1 (fluoroalkyl acid) to 1720 cm.sup.-1 shows the formation of the
amide linkage.
EXAMPLE 1b
Fluoroether Versions of 1a
Fluoroether carboxylic acids (Krytox functional fluids, DuPont, FSL=2500
molecular weight, FSM=5000 molecular weight, FSH=7500 molecular weight)
were first transformed to their respective acid chlorides. For example,
the oligomer of hexafluoropropylene oxide, capped at one end with a
carboxylic acid group (7500 molecular weight) was transformed to the acid
chloride via reaction with thionyl chloride. In a typical reaction, 30 g
of 7500 molecular weight fluoroether (4 mmol) and 50 ml. of previously
dried perfluorol, 3-dimethyl cyclohexane were added to a reaction flask
equipped with a condenser. Subsequently, 0.95 g of thionyl chloride (8
mmol) and 0.58 g of dimethylformamide (8 mmol) were added and the mixture
was heated at reflux under a blanket of nitrogen. The residual reactants
and DMF are removed via extraction in ether. The solvents in product are
removed under vacuum at 75-80.degree. C. The product is characterized by
the disappearance of the carboxylic acid peak at 1777 cm.sup.-1 and the
appearance of the acid chloride peak at 1810 cm.sup.-1 on the FT-IR
spectrum and also by the disappearance of the COOH proton at 9.6 ppm on
the .sup.1 H NMR spectrum.
The fluoroether acid chlorides thus generated are used to synthesize
fluoroether-functional tertiary amines as described in 1a, where the
fluoroether acid chloride is employed in place of perfluorooctanoyl
chloride.
EXAMPLE 1c
Analogs to (1a) With Two CO.sub.2 -Philic Tails
Versions of 1a and 1b with two CO.sub.2 -philic tails are synthesized by
reacting 3,3'-diamino-N-methyldipropylamine with either perfluorooctanoyl
chloride in a 2:1 molar ratio using the same conditions as in 1a, or with
one of the fluoroether acid chlorides as in 1b.
EXAMPLE 1d
Analogs to (1a) With Three CO.sub.2 -Philic Tails
Versions of 1a and 1b with three CO.sub.2 -philic tails are synthesized by
reacting tris(2-aminoethyl)amine with either perfluorooctanoyl chloride in
a 3:1 molar ratio using the same conditions as in 1a, or with one of the
fluoroether acid chlorides as in 1b.
EXAMPLE 2a
Measurement of Binding of Tertiary Amine Extractants with Platinum
Hexachloride
H.sub.2 PtCl.sub.6 was purchased from Aldrich Chemical Co. and dissolved in
0.2M HCl at 1 mg/ml. At the same time, various amounts of the
single-tailed fluoroalkyl tertiary amine (1a) were dissolved in 1,1,2
trichlorotrifluoroethane. The two solutions were contacted at an
organic:aqueous v/v ratio of 3:7 for several hours while stirring.
Following separation of the phases, the concentration of platinum
remaining in the aqueous phase was measured using the UV method of
Marczenko [9], and compared with the initial level. results are shown
below:
Agent:Pt
molar ratio % extracted
0.395 9.05
1.581 45.03
2.767 59.84
3.953 97.88
which shows that the tertiary amines retain their ability to bind platinum
anions from solution after functionalization with CO.sub.2 -philic tails.
EXAMPLE 2b
The experiment as shown in example 2 except that the fluoroether-functional
single tail tertiary amine (1b, molecular weight of 5000) was used.
Agent:Pt
molar ratio % extracted
.88 15.42
3.52 45.03
6.16 76.14
8.80 82.89
which shows that the fluoroether-functional amines can also bind platinum
from aqueous HCl solution.
EXAMPLE 3
Solubility of Complexes of Platinum Hexachloride with CO.sub.2 -Philic
Extractants in CO.sub.2
Phase behavior studies of the metal binding group/metal complexes in carbon
dioxide were conducted using a high-pressure, variable-volume view cell
(D. B. Robinson and Associates). Typically, a known amount of sample
(0.3-1.0 g) was added to the top of the quartz tube sample cell along with
a number of glass or steel ball bearings to provide mixing. The tube was
then sealed inside the steel housing, and a known volume of carbon dioxide
was injected into the cell using one of the two Ruska syringe pumps. The
quartz sample tube contains a floating piston that separates the sample
from the pressure-transmitting fluid, in this study, a silicone oil. The
pressure on the sample was raised (via the movement of the piston due to
injection of silicone oil by the second Ruska pump) to a point where a
single phase was present. Mixing was accomplished by the motion of the
ball bearings upon rocking of the entire cell. The pressure was then
lowered via slow withdrawal of silicone oil from beneath the piston until
the first sign of turbidity appeared, which was indicative of a phase
separation. This procedure was repeated until the point of turbidity was
known to within 20-30 psi. The corresponding point on the pressure vs.
concentration curve was identified as a cloud point. Following
identification of the cloud point, an additional amount of measured carbon
dioxide was injected into the cell to obtain a new concentration of the
chelating agent. The cloud point for this new concentration was measured,
and this procedure was repeated until the entire cloud point curve was
completed.
Complexes of platinum hexachloride with both the fluoroalkyl-functional
tertiary amine and the fluoroether-functional tertiary amine were formed
via contact between aqueous and organic solutions. An amount of H.sub.2
PtCl.sub.6 was dissolved in 0.2 M HCl; an amount of the extractant in
question was dissolved in 1,1,2 trichlorotrifluoroethane such that
platinum was in considerable excess. The two solutions were contacted with
vigorous stirring for 24 hours. After separation of the phases, UV
analysis of the aqueous phase showed that the extractant had bound
platinum. Consequently, the organic phase was retained and the solvent
removed to recover the platinum complex, which was then examined in the
Robinson cell as described above.
EXAMPLE 4
A series of effective platinum-binding extractants was made, in which the
extractants had the general formula
##STR1##
where R is --(CF.sub.2).sub.m CF.sub.3 where m is preferably 2 to 10 and
more preferably about 7, or --(CF.sub.2 CF(CF.sub.3)O).sub.n CF.sub.2
CF.sub.2 where n is preferably 2 to 25 and more preferably about 14. In
the exemplary formulations, x was 3, y ranged from 1 to 3, and z was 3.
The subscripts x and z can vary greatly, but each will typically be less
than 25 and more preferably less than 10. The extractants made according
to formula (1) were each found to bind to platinum and gold to at least a
measurable degree.
The foregoing experiments show that it is possible to solubilize platinum
complexes of CO2-philic amine extractants in CO2.
EXAMPLE 5
It is expected that extractants having the formula
R--(CH.sub.2).sub.x --S--(CH.sub.2).sub.y --R' (2)
were R is a CO.sub.2 -philic group such as a fluoroalkyl, a fluoroether, or
silicone as described above, and R' can also be a CO.sub.2 -philic group,
or an alkyl (CH2).sub.z group (i.e., either R or R' or both can be
CO.sub.2 -philic). In this embodiment, x and y are each preferably greater
than or equal to 2, and more preferably 2-4.
EXAMPLE 6
It is further believed that extractants having the formula
R--(CH.sub.2).sub.x --R' (3)
where R is a CO.sub.2 -philic group such as a fluoroalkyl, a fluoroether,
or silicone as described above, and R' is an ether, ester, ketone, or
carboxylic acid group, and x is greater than or equal to 2, preferably
2-4. The oxygen-bearing compounds such as ketones, ethers, etc., bind to
gold, but not to platinum or palladium. Hence extractants made according
to formula (3) are particularly effective for gold extraction.
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